Gravity Lenses | Einstein Cross | Rings | Arcs

Albert Einstein published his General Theory of Relativity in 1916. One aspect of the theory predicted that light passing near a massive object would be deflected slightly in direction. Light originating from a distant bright source such as a quasar could travel by a massive object such as a galactic center and change direction. The observer would see the image of the distant source in slightly different places.

Not to scale.

Of course, such a prediction deserved to be tested. In 1919, astronomer Arthur Stanley Eddington headed a team to observe a star closely aligned with the Sun that would be visible only during a total eclipse. The star’s known position should be shifted according to the theory. Their observations confirmed that Einstein was correct.

This effect of changing the direction of light by intervening mass is known as gravitational lensing. It has been observed and confirmed many times. This example called Huchra’s Lens was discovered in 1985 by John Huchra. The galaxy is in the foreground. It’s core is aligned with a quasar ten times farther away. The light from the quasar passing the massive core is forming four images of the distant quasar. The box defines a region imaged by the Hubble telescope seen after this image.

European Southern Observatory| F. Courbin et al.

This higher resolution Hubble image of the boxed area above is a composite I made from the Hubble Legacy Archive using 3 greyscale images. The distant quasar and its four gravitationally lensed images are clearly visible.

Hubble Legacy Archive | My version

Gravitational lens arrangements can also cause a distant quasar to appear as a ring around a less distant galaxy. Here are several examples captured by the Hubble telescope.

NASA/ESA/SLACS Survey Team: A. Bolton (Harvard/Smithsonian), S. Burles (MIT), L. Koopmans (Kapteyn), T.Treu (UCSB), L. Moustakas (JPL/Caltech)

I used a small light source and bottom of a small juice glass to simulate the formation of a ring and arc of light.

Gravitational lensing can also form arcs as images of distant objects. Many examples are documented. This image of the region Abell 2218 is a cluster of galaxies about 2 billion light-years away. These nearer galaxies are spiral and elliptical in shape and appear as fuzzy blobs. Much more distant galaxies aligned with 2218 have been distorted into arcs by the intervening masses of the galaxies. Some of that mass is in the form of dark matter and invisible to our equipment. But, it has gravitational influence on passing light.

Abell 2218 | NASA | ESA


16 thoughts on “Gravity Lenses | Einstein Cross | Rings | Arcs

  1. Good summary – but one thing that has always bothered me about Einstein Rings is that they always seem to be seen as narrowly defined halos or arcs.

    Looking at the light-flow diagram at the top of your post, just as a thought experiment, if the intermediate galaxy were imagined as a gigantic glass lens, then the distant quasar would appear as an enlarged conterminous object, centred in its actual location. There would be no ring structure as such, just a visible magnification of the quasar.

    So, what causes the halo? Obviously the galaxy structure impedes the centre path to the quasar and that needs to be considered but it remains unclear to me why the rings are so clearly defined, especially at the inner edge.

    I suspect the answer lies either: (1) somewhere deep inside General Relativity 😨 or (2) is just the simple geometry of space triangles and we are not seeing the centre of the magnified quasar because the light following that path has already passed us by.

    • Roger, you bring up some good questions. I would like to hear responses of someone who is trained in GR. I know someone to ask. But, don’t hold your breath for lots of understandable answers. 🙂

    • I added a video simulation to the post you might appreciate. It is my attempt to illustrate something said in a comment below yours by Jim Wheeler about imperfect lenses and ring and arc formation.

  2. I don’t recall reading before that gravitational lensing caused by dark matter has been observed. That is very interesting and helps me believe dark matter exists. Theory is one thing, but actual evidence is another.

    As for Greybeard’s question, I submit that the arcs are just the result of the variability of the structure of the intervening lensing galaxy. It is an imperfect lens, in other words.

    • A lot like lookin down through a wine glass at a light 💡 source. Much distortion occurs giving various patterns of brightness.

  3. Thanks for the added video.

    I contacted a Cosmologist at Sydney University, Prof. Geraint Lewis, who is Patron of our astronomical society. He always seems willing to respond to (sensible) questions.

    He independently confirmed both Jim Wheeler’s “imperfect lens” and that as you stated – gravitational lensing is analagous to a “wine glass” effect. His response (in part) was:

    “In the same way that a bathroom mirror gives you a nice image, but a fairground mirror gives you a horribly distorted, multiple image version, a gravitational lens (made by nature) does not have the optical properties of a nicely made lens for your eye. In fact, given that galaxies have lots of mass at their centre and less at the outskirts, it’s more equivalent to using the base of a wine glass as a lens”

    He also sent me this interesting link which also demonstrates the wine glass:

    My next glass of red wine could be a messy affair……


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